Metal extraction prior to chelation in chemical pulp production

ABSTRACT

The amount of metals that can be removed from comminuted cellulosic fibrous material (such as wood chips) in the production of chemical pulp, prior to bleaching, is enhanced by extracting metals during an early stage of digestion, and prior to the addition of a chelating agent. After the material is steamed and slurried it is impregnated with cooking liquor at a temperature of 90° C. or more, and during or after impregnation metals are removed by an extraction (e.g. in a continuous digester), which typically removes about 30% or more of the Mn. Prior to this extraction it is not necessary to introduce chelating agents, and undesirable to do so since they may compete or interfere with the natural removal of metals. After the extraction, such as during the first part of cooking, about 0.05-10 kg/dry ton of material of chelant (such as EDTA) is added, and the chelant combines with released metal ions in the slurry to produce metal complexes. The metal complexes are substantially removed before bleaching. In this way more than 93% of Mn (and other undesirable metal ions) may be removed prior to bleaching with a minimal amount of chelating agent used.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of application Ser.No. 08/659,682 filed Jun. 5, 1996, the disclosure of which is herebyincorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] It is known that the removal of metals from chemical pulp beforeit reaches the bleach line increases the efficiency of the bleachingprocess, especially when bleaching chemicals, such as peroxide, whichare very adversely affected by the presence of metal ions (particularlytransition metal ions) are present. In the present application, it hasbeen recognized that the addition of a chelating agent to a digester(such as a continuous digester) during the digestion process enhancestransition metal ion removal since the chelant combines with releasedtransition metal ions in the slurry to produce metal complexes, whichare subsequently removed during the cooking process. It has now beenfound that one of the particular embodiments disclosed in the parentapplication which is the most advantageous for removing the maximumamount of metal ions with a minimum amount of chelating agent is to addthe chelating agent (metal-complex former) downstream of a liquorextraction, or after or during the process of extracting and dilutingthe cooking liquor. This chelate treatment may be practiced in aco-current or counter-current cooking mode, in a single or multiplevessel digester system, in a hydraulic or dual phase digester, or inbatch or continuous fashion.

[0003] According to the present invention, the disclosure in the parentapplication with respect to adding the chelating agent after anextraction has been amplified upon and optimized, while still retainingthe same basic concept. That is, during the digestion process, but afteran extraction of metals that was obtained without the use of a chelatingagent, the chelating agent is added. It has been found according to thepresent invention that if a wood chips slurry is simply treated bysteaming and alkali impregnation (the first stage in the digestionprocess) at a temperature of about 90° C. or more, without the additionof chelants, a significant amount of the undesirable metals, such as Mn,may be removed. For example according to several procedures performedaccording to the invention, more than 37% of the Mn originally presentin the chips can be removed just by conventional steaming and alkaliimpregnation. If—as according to the present invention—the chelatingagent is then added after an extraction of these metals (typically withblack liquor through a conventional extraction screen), then the totalmetal removal capability is improved. That is, it has been found that itis not necessary to introduce chelants prior to impregnation, or duringimpregnation, where they may compete or interfere with the naturalremoval of metals; but rather the chelants may be reduced later duringthe digestion process, typically after impregnation and at the start ofcooking, so that a more efficient, less costly use of chelants isprovided.

[0004] According to one aspect of the present invention there isprovided a method of producing chemical pulp from comminuted cellulosicfibrous material containing metal containing compounds comprising: (a)Pressurizing and slurrying the comminuted cellulosic fibrous materialcontaining metal containing compounds, and heating the slurry. (b)Digesting the material in the slurry, including in different stages, andby impregnating the material with cooking liquor, and cooking thematerial to produce chemical pulp. (c) At an early stage duringdigestion, extracting liquid containing some of the metal containingcompounds therein, and removing the metal containing compounds from theslurry. (d) After (c), adding a chelating agent to the slurry to combinewith released metal containing compounds in the slurry to produce metalcomplexes; (e) cooking the material; and (f) removing at least some ofthe metal complexes formed in (d). Metal containing compounds includemetal ions, metal oxides, and any other metal-containing compound thatis preferably removed from the slurry.

[0005] The method further comprises, after (f), bleaching the chemicalpulp with at least one bleaching chemical adversely effected by at leastsome of the metal ions removed in (c) and (f), such as peroxide.Typically (c) is practiced to remove at least about 30% of the Mn in thematerial, from the material, and (c) and (f) are practiced to remove atleast about 90% of the Mn, e.g. at least about 93%, as well as themajority of at least the other transition metals originally present inthe material, such as wood chips.

[0006] Typically (c) is practiced after impregnation and just beforecooking, at a temperature of between about 105-145° C. (e.g. about110-130° C.), for a time period of between about 30-120 minutes,preferably between about 60-120 minutes (e.g. about 90 minutes), andwith a charge of chelating agent of between about 0.05-10 kg/dry ton ofmaterial, preferably about 1-5 kg/dry ton of material, most preferablyabout 1-2 kg/dry ton of material. Typically (d) is practiced where thepH is 9 or more, although the invention can be practiced at a widevariety of pHs. The chelating agent may be selected from the groupconsisting essentially of EDTA, DTPA, derivatives and equivalents toEDTA and DTPA, oxalic acid, tartaric acid, and furoic acid; howeveralmost any metal complex former may be utilized, preferably one that istemperature resistant (preferably one that can even withstand thetypical cooking temperatures of about 150° C.), and one that canwithstand high pH (that is a pH of greater than 9, typically greaterthan 10, preferably greater than about 12). In any event, enoughchelating agent is used to remove at least 10% of the transition metalions in the material, preferably at least the majority of the transitionmetal ions in the material, from the material.

[0007] According to another aspect of the present invention there isprovided a method of producing chemical pulp from comminuted cellulosicfibrous material containing metal ions comprising: (a) Pressurizing andslurrying the comminuted cellulosic fibrous material containing metalcompounds, and heating the slurry. (b) Impregnating the material withcooking liquor at a temperature of about 90° C. or more. (c) After (b),extracting liquid containing some of the metal compounds therein, andremoving the metal ions from the slurry. (d) After (c) adding achelating agent to the slurry to combine with released metal compoundsin the slurry to produce metal complexes; (e) cooking the material; and,(f) removing at least some of the metal complexes formed in (d). Thedetails of the method may be as described above.

[0008] In practicing the method (a)-(f) may be practiced in an uprightvessel having an upper screen and a black liquor extraction screen; andwherein (d) is practiced by adding chelant in a first conduit at a firstdistance below the upper screen, and above the extraction screen, sothat the chelant at least primarily flows with liquid up through theslurry to the upper screen, and wherein (e) is practiced in part byintroducing cooking liquor in a second conduit at a second distancebelow the upper screen, greater than the first distance, so that thecooking liquor flows with the slurry downwardly toward the extractionscreen.

[0009] The methods according to the present invention are preferablypracticed substantially continuously, e.g. using conventional continuousdigesters including all different types (one vessel, two vessels,hydraulic filled, gas phase, etc.). However, the invention is alsoapplicable to batch digestion in the production of pulp. The inventionis particularly suitable for use in kraft pulping, although other typesof chemical pulping, including sulfite and soda pulping, also benefitsignificantly from the practice of the invention.

[0010] The invention also relates to the chemical pulp produced by thepractice of the method as described above.

[0011] There is the possibility of hexenuronic acid (HexA) formationduring the alkaline pulping process and there is the potential for theseacids to act as charged sites on the cellulose to which charged metalions can attach. According to one aspect of the present inventiondisclosed herein, this metal ion attachment to the pulp via charged HexAcan be minimized by treating the pulp toward the end of the cook with ahighly alkaline liquor. However, there is another aspect of the presentinvention that also pertains to when in the process the HexA isgenerated.

[0012] It is understood that HexAs are generated in the alkaline pulpingprocess at temperatures at or above 100° C., and the problem is mostsignificant at or above 150° C., for example, at or above 160° C. Theseelectrically charged HexAs can attract charged metal ions. However,according to the preferred embodiment of this invention, at least some,if not most of the charged metal ions are removed, for example, viapost-impregnation extraction or via chelate treatment, prior to the pulpbeing exposed to temperatures which generate HexAs. That is, anotherbenefit of the present invention, in which metal ions are removed afterimpregnation or during chelate treatment, is that these metals areremoved before they can be exposed to HexAs generated later in the cook.

[0013] Thus the invention also relates to a method as described abovewherein the chelant is introduced prior to the slurry achieving atemperature at which the generation of an adversely significant amountof hexenuronic acids beings, i.e. prior to achieving a temperature ofabout 140° C. or more (or under some circumstances about 120° C., oreven about 100° C., or more).

[0014] It is the primary object of the present invention to provideeffective removal of transition metal ions from comminuted cellulosicfibrous material, such as wood chips, prior to bleaching, and with aminimum amount of chelant. This and other objects of the invention willbecome clear from an inspection of the detailed description of theinvention, and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic view illustrating an exemplary methodaccording to the present invention;

[0016]FIG. 2 is a schematic view showing an exemplary continuousdigester system which may be utilized for the practice of the presentinvention; and

[0017]FIG. 3 is a view like that of FIG. 2 showing another exemplarysystem which may be utilized in the practice of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 schematically illustrates one embodiment of a methodaccording to the present invention. Cellulosic fibrous material, such assoftwood or hardwood chips in line 1, is steamed at 2 to remove excessair and to begin heating the material. The steaming at 2 may beperformed using any conventional techniques, preferably with aDIAMONDBACK® steaming vessel, as described in U.S. Pat. Nos. 5,500,083,5,617,975, and 5,628,873, and such as sold by Ahlstrom Machinery. Aftersteaming, cooking liquor (e.g., white liquor, black liquor, sulfitecooking liquor, etc.) 3 is added to the material to form a slurry, andthe slurry is pressurized, as schematically illustrated at 4. Slurryingand pressurizing may be performed in a conventional high pressurefeeder, or by a slurry pump and feeder as described in U.S. Pat. No.5,476,572, or by one or more pumps as described in U.S. Pat. No.5,753,075, all sold by Ahlstrom Machinery.

[0019] The cellulose material slurry is then heated at 5 to atemperature at which transition metal compounds (e.g. ions) dissociatefrom the material. This temperature is at least about 90° C., preferablyat least about 100° C., typically at least about 110° C., which in theembodiment illustrated is in the impregnation stage of the digestionprocess. With some of the transition metal ions released from thematerial (typically in solution), extraction of the metal ions occurs asillustrated schematically at 6 in FIG. 1, producing a liquid flow 7containing the metals, which is typically removed entirely from theslurry (that is, not recirculated, although after metal ion removal fromthe liquid, the liquid itself can be returned, or any desirable metalions, such as Mg or Ca removed from the liquid, can be added back to thematerial). The extraction at 6 can be accomplished utilizing theconventional screens and like structures that are common in both batchand continuous digesters.

[0020] After the liquid containing the metals in line 7 is removed—whichtypically takes with it at least about 30% of the Mn, and alsosignificant amounts of other undesirable metal ions—the slurry issubjected to a chelant treatment, as illustrated at 8 in FIG. 1. Thechelant treatment 8 typically takes place at the same consistency as therest of the pulping process, that is preferably medium consistency, e.g.about 8-20%. The relationship of the relative amount of material andliquid present is typically expressed by the liquid-to-material, orliquid-to-wood (UW) ratio. The UW ratio for the chelate treatment 8typically ranges between 1.0-5.0, preferably about 2.0 to 4.0.

[0021] A chelating agent is added at one or more points 9 to the slurry,and typically the slurry is maintained at a relatively low temperature,for a significant period of time, so that the chelating agent combineswith released metal ions in the slurry to produce metal complexes.Preferably the chelating agent treatment 8 takes place for a time periodof between about 30-120 minutes, preferably about 60-120 minutes, andmost preferably about 90 minutes, at a temperature of about 105-145° C.,preferably between about 105-125° C., e.g. about 110° C. While thisrelatively low temperature range is preferred, a higher temperaturerange may be used if it suits the process, all the way up to thetemperature at which the chelating agent may no longer be operable.Typically the pH during the treatment 8 is nine or more, normally ten ormore, and usually at least about twelve, although the metal complexingaction will take place even at much lower pHs.

[0022] The amount of chelant added at 9 is whatever is effective toremove the desired amount of metal ions. Typically the charge is betweenabout 0.05-10 kg/dry ton of material, particularly when EDTA or itsequivalent is utilized. Normally the range is between about 1-5, andmost preferably is relatively low, e.g. about 1-2 kg/dry ton ofmaterial. Although any agent which complexes metal-containing compoundsmay be used in the practice of this invention, it is preferred that theagent used be stable under the conditions described, that is, the agentis preferably stable under alkaline conditions at temperatures of 100°C. and above. For example, preferred metal-complexing agents includenitrogen-containing polycarboxyl compounds, such asethylenediaminetetraacetic acid (EDTA), anddiethylenetriaminepentaacetic acid (DTPA), or their salts andderivatives, as well as hydroxycarboxyl compounds, such as citric andtartaric acids, or their salts and derivatives.

[0023] Cooking liquor, such as white liquor, may be added in one or bothof the lines 10 to the slurry, and after the chelating treatment, thematerial is heated to cooking temperature, of at least about 140° C. andtypically between 160-170° C., and digestion is completed by one or morecooking stages illustrated schematically at 11 in FIG. 1. During thecooking stage 11, and in fact even during the chelant treatment 8,liquid may be extracted containing high levels of dissolved organicmaterial, and replacement liquors containing lower amounts of dissolvedorganic material (such as white liquor, water, filtrate, or the like)added, as illustrated schematically at 12 in FIG. 1.

[0024] At least some of the formed metal complexes are removed,schematically illustrated at 13 in FIG. 1, with black liquor during thecooking at 11, or in the subsequent wash 14, or at any other suitablepoint prior to bleaching at 15. Preferably the vast majority of metalcomplexes are removed, e.g. more than 90%.

[0025] After cooking, the chemical pulp produced is washed, asillustrated schematically at 14 in FIG. 1, and subsequently (typicallyafter storage and screening and perhaps additional washing) is bleached,as schematically illustrated at 15 in FIG. 1 preferably TCF (totallychlorine free) bleaching, or ECF (elemental chlorine free) bleaching,typically including at least one stage of peroxide or ozone.

[0026] All of the stages 5 through 14 illustrated in FIG. 1 may takeplace in a conventional continuous digester, utilizing conventionalprocesses such as EMCC® as disclosed in European patent 476,230 B1,Lo-Solids® treatment, such as disclosed in U.S. Pat. Nos. 5,489,363,5,547,012, 5,620,562, and 5,849,150 (the disclosures of which are herebyincorporated by reference herein), or other suitable pulping processesmay be utilized.

[0027] By practicing the invention as described above, an enhancedamount of metal ions can be removed, for a given amount of chelatingagent—than in known processes. For example in the practice of theinvention compared to the procedure set forth in U.S. Pat. No. 5,593,544where the chelating agent (called a sequestering agent there) is addedin a pre-impregnation stage, prior to the actual digestion process (thatis prior to impregnation and cooking), according to the invention 2% ormore of the amount of Mn, and enhanced amounts of other adverse metalions, can be removed with a given amount of chelating agent.

[0028]FIG. 2 schematically illustrates a continuous digester system thatmay be utilized to practice the method of FIG. 1. In FIG. 2, wood chips,or like comminuted cellulosic fibrous material, are fed to a chip bin 20(such as a DIAMONDBACK® chin bin) where preferably steaming takes place(although steaming may be in a steaming vessel located downstream of thechip bin 20), and then the chips are pressurized and slurried as in aconventional chip tube 21 or like conventional equipment. Then a highpressure feeder or slurry pump, the high pressure feeder beingillustrated in FIG. 2 schematically at 22, feeds the pressurized slurryin line 23 to the inlet 24 of a continuous digester 25. Cooking liquorhas already been added, as indicated at 25 in FIG. 2, and the liquid inline 23, which is recirculated in line 26, is hot, over 90° C. At thetop of the digester 25 impregnation occurs, typically at a temperatureof between about 105-145° C., and then utilizing a conventional screen27 or the like, metals extraction occurs, as illustrated schematicallyat 28. Then a chelant is added, as illustrated schematically at 29 inFIG. 2, and approximately in zone 30 chelant treatment occurs, typicallyfor a time period of about 60-120 minutes at a temperature of about105-145° C. with a chelant charge of between about 0.05-10 kg/dry ton ofpulp, and at a pH of over 9. A recirculation line 31, and heater 32, maybe provided, and some of the liquid (typically high DOM liquid) may bewithdrawn, as schematically illustrated at 33 in FIG. 2.

[0029] Black liquor, and metal complexes, can be removed as illustratedat 34 in FIG. 2, using a conventional extraction screen 35 or the like,and one or more cooking stages, illustrated schematically at 36 in FIG.2, are provided, as well as a conventional wash/cook stage 37. Cookingstages 36 may also be omitted, if desired. Conventional recirculations38, 39 are also provided, along with conventional heaters 40, 41, andsome liquor may be extracted (such as high DOM liquor) as illustratedschematically at 42 and 43, and replaced with low DOM liquor. Whiteliquor and/or low DOM dilution liquor may also be added at variouspoints, as also illustrated schematically at 44 and 45 in FIG. 2.

[0030] According to the present invention actual cooks were madeutilizing both softwood and hardwood chips to demonstrate the advantagesof the invention.

[0031] In these tests, the softwood furnish used was Northern softwoodfrom an Ontario mill (see Table 1). The Southern hardwood furnish camefrom a Southeastern U.S. mill. Treatment of the softwood was completedin five liter electrically heated vessels that swung through a 270° arcto maintain mixing and uniform temperature. For each treatment 750 gramsof oven-dry chips were steamed for 15 minutes at 100° C. and then placedin the vessel. A synthetic white liquor charge with an Effective Alkali(EA) of 9.0% on wood and a sulfidity of 30% based on Active Alkali (AA)nominal was added at a liquid to wood ratio of 3.5 Ukg. There was a 15minute rise to impregnation temperature of either 95° C. or 110° C.Impregnation was maintained for 30 minutes. 850 mL of free liquor wasdrained from the digester at the end of impregnation. A white liquorcharge with an EA of 3.0% on wood equal in volume to the drained liquorwas added. This charge did or did not include 1.0% on wood EDTA chargeas required. The time from end of impregnation to the restarting of theheating was about 18 minutes. Heating was continued to bring the vesselback to the impregnation temperature. The time to reheat, about nineminutes, plus the time at temperature was either 45 or 90 minutes.

[0032] The liquor was rained after the treatment stage and the treatedchips washed in the vessel for ten minutes with deionized water. Thechips then underwent a displacement wash overnight with deionized water.The chips were drained for about two hours and a sample taken foranalysis. The end of impregnation and treatment stage black liquors wereanalyzed for residual effective alkali. The liquors and the treatedchips were analyzed for the metals calcium, copper, iron, magnesium, andmanganese. The treatment conditions are listed in Table 2.

[0033] Conventional kraft cooking of the softwood chips, followingPruyn's Island Technical Center (PITC) standard operating procedurePITC-P2, then took place. One reference and one cook with 1.0% on woodEDTA added at impregnation were completed. The black liquors wereanalyzed for residual effective alkali. The liquors and pulp wereanalyzed for the metals calcium (Ca), copper (Cu), iron (Fe), magnesium(Mg), and manganese (Mn). The cook conditions are given in Table 3.

[0034] Laboratory Lo-Solids® cooking was then practiced, followingprocedure PITC-P27. One reference cook and one with a 1.0% on wood EDTAcharge at impregnation were completed.

[0035] A third cook used a modification of the Lo-Solids® procedure,according to the invention. After impregnation, 3966 mL of black liquorwas drained from the digester. An equal volume of white liquor with anEA of 3.0% on wood was added along with 1% on wood EDTA. This processtook 17 minutes. The treatment stage was 90 minutes at 110° C. The cookcontinued with the standard Lo-Solids® procedure at the end of thetreatment stage.

[0036] The black liquors were analyzed for residual effective alkali.The liquors and pulp were analyzed for the metals calcium, copper, iron,magnesium, and manganese. The cook conditions are given in Table 3.

[0037] A laboratory Extended Modified Continuous Cook (EMCC®) wascompleted with an EDTA charge of 1.0% on wood added to the impregnationstage. This cook followed procedure PITC-P3. The black liquors wereanalyzed for residual effective alkali. The liquors and pulp wereanalyzed for the metals calcium, copper, iron, magnesium, and manganese.The cook conditions are given in Table 3.

[0038] Thus, as shown in Table 2, in these trials the effect of thepresence of 1% EDTA chelating agent (which is approximately 10.0 kg/tonof dry softwood chips), and of varying impregnation and treatmenttemperatures and times were compared to a conventional kraft cook (CK)without any chelant added, that is cook S1159. Note that these trialscompared the results for impregnation and chelate treatment only. Thechips were not subsequently cooked. The chips were first impregnatedwith alkali at time and temperature and then treated with chelant andalkali at time and temperature.

[0039] The resulting metal content of the chips treated in these testsappear in Table 5. As shown therein, two samples were analyzed for metalcontent for each treatment, and the mean and standard deviation for themetal content were calculated from the two sets of sample data.

[0040] The treatment conditions listed in the upper sections of Table 2and the mean metal content that appears in Table 5 are summarized inTable 7. The metal content of the untreated chips (“Orig. Chips”)appears in the first column. Cook S1159 corresponds to the impregnationaccording to a conventional kraft cook without any chelate added. If themetal content is limited to Mn, which is known to exhibit the mostsignificant cellulose damage, especially during non-chlorine bleaching,Table 7 shows that the conventional impregnation reduces the Mn contentby about 37%. This shows that it is not necessary to introduce chelantsearly in the process where they may compete or interfere with thenatural removal of metals.

[0041] In Cook S1160, the same conditions as Cook S1159 were used exceptthat S1160 includes a treatment with chelating agent duringimpregnation. This resulted in Mn removal of 69%. In cook S1161, thetreatment time with the chelant was extended from 45 to 90 minutes. Asshown, the Mn removal only increased from 69 to 76% with the extratreatment time. In cook S1162, the chelant treatment time was againreduced to 45 minutes, but the treatment temperature for both theimpregnation and the chelant treatment was increased from 95 to 110° C.However, compared to the lower temperature treatment, cook S1160, themetal removal actually decreased to 66%. In cook S1163, both thetreatment time was increased from 45 to 90 minutes and the impregnationand chelant treatment temperature was increased from 95 to 110° C. Thisproduced a markedly increased removal of Mn, 89%, and also a markedlyincreased removal of calcium (Ca) and magnesium (Mg) compared to theother treatments.

[0042] The result of these impregnation and chelate treatment trials wasthe finding that increased metal removal can be achieved when treatingthe chips at a higher temperature, for example, 110° C. instead of 95°C., for a longer treatment time, for example, 90 minutes instead of 45minutes. This increased metal removal at higher temperature and longertime during pretreatment provided a basis for the further investigationof the impact of the presence of chelants on the metal content of thefully cooked pulp.

[0043] The cooking test conditions for the Northern softwood cooksappear in Table 3. Employing the results of the earlier treatment tests,the impregnation and chelate treatment, when applicable, were performedat the higher temperature, that is, at 110° C.

[0044] Cook S2222 corresponds to a conventional kraft cook withoutchelate addition. Cook S1182 corresponds to a conventional kraft cookwith chelate addition, that is, the chips are treated with 1% EDTA onwood in a treatment stage after impregnation with alkali at time andtemperature. Cook S1182 is similar to the process disclosed in U.S. Pat.No. 5,593,544 in which the chelant is added during a pretreatmentwithout any liquor removal before chelant addition. Cook AL622corresponds to a Lo-Solids® cook without chelate addition. Thislaboratory batch simulation includes a small liquor purge, or removal,(about 4 liters of liquor) during the counter-current (or displacement1) stage to simulate a removal of liquor containing dissolved organicmaterial and dissolved metals as is characteristic of Lo-Solids cooking.Cook AL624 is a Lo-Solids® cooking simulation similar to cook AL622 butwith the addition of 1% EDTA during the impregnation stage. Cook AL624also included a small liquor purge to simulate Lo-Solids®) cooking as incook AL622. Cook AL625 is also a Lo-Solids-type cook with the additionof 1% EDTA, but according to the present invention, in cook AL625, afterimpregnation (without the presence of chelant), black liquor was drainedfrom the vessel. After this liquor removal, alkali and EDTA wereintroduced to the digester. That is, test AL625 corresponds to theinvention, and includes a post-impregnation extraction after which thechips are treated with a chelant and alkali for 90 minutes at 110° C.

[0045] Cook AE647 in Table 3 is a simulation of a conventionalEMCC®-type cook with chelant added to the impregnation stage. Cook AE647with chelant added during impregnation and no liquor removal beforeformal cooking is commenced is similar to Test 9 of U.S. Pat. No.5,593,544.

[0046] The results of the testing shown in Table 3 are also shown inTable 5. Table 5 also includes the results of metal analysis for thepulp. The metal content of the untreated chips for the tests performedaccording to Table 3 is the same as the metal content shown for thechips. The testing conditions shown in Table 3, and the results shown inTable 5, are summarized in Table 8. As shown, again with reference to Mnremoval, the cook performed according to the present invention (that is,cook AL625) removed the most Mn, 94%. The cook according the presentinvention removed more Mn than the cooks that corresponded generally towhat is disclosed in U.S. Pat. No. 5,583,544, that is, more than cookS1182 (only 81%) and cook AE647 (91%).

[0047] Other tests were performed on Southern hardwood (see the bottomof Table 1). Treatment of the hardwood was completed in five literelectrically heated vessels that swung through a 270° arc to maintainmixing and uniform temperature. For each treatment 750 g of oven-drychips were steamed for 15 minutes at 100° C. and then placed in thevessel. A synthetic white liquor charge with an EA of 9.0% on wood and asulfidity of 30% based on AA nominal was added at a liquid to wood ratioof 3.5 Ukg. There was a 15 minute rise to impregnation temperature of110° C. Impregnation was maintained for 30 minutes. 850 mL of freeliquor was drained from the digester at the end of impregnation. A whiteliquor charge with an EA of 3.0% on wood equal in volume to the drainedliquor was added. This charge did or did not include a 0.5 or 1.0% onwood EDTA charge as required. The time from the end of impregnation tothe restarting of the heating was about 18 minutes. Heating wascontinued to bring the vessel to 110° C. or 130° C. The time to heat,about nine minutes, plus the time at temperature was 90 minutes.

[0048] The liquor was drained after the treatment stage and the treatedchips washed in the vessel for ten minutes with deionized water. Thechips then underwent a displacement wash overnight with deionized water.The chips were drained for about two hours and a sample taken foranalysis. The end of impregnation and treatment stage black liquors wereanalyzed for residual effective alkali. The liquors and the treatedchips were analyzed for the metals calcium, copper, iron, magnesium, andmanganese. The treatment conditions are listed in Table 4.

[0049] That is, the treatment of hardwood chips was essentiallyidentical to the treatment of the softwood chips discussed earlier.Again, the treatment conditions listed in Table 4 are for animpregnation and chelate treatment only, not a cook. The resulting metalcontent of the treated chips are shown in Table 6. The treatmentconditions and resulting metal content are summarized in Table 9.

[0050] The conditions shown in Table 9 employ the findings of thetesting performed on the softwood chips shown in Table 8. That is, thechelant treatment time is longer, 90 minutes, and the treatmenttemperature is higher, 110° C. Furthermore, in the last two trials(S1170 and S1171) the treatment temperature was even higher, that is,130° C. These trials also included a reduction in the chelantconcentration from 1.0% to 0.5% on wood.

[0051] Cook S1166 is the reference cook with no chelate addition duringpretreatment. Again, as in the earlier tests on softwood, this testindicates that as much as 32% of the Mn can be removed from the chipswithout the use of a chelant. Cook S1168 is similar to cook S1166 butwith the introduction of 0.5% EDTA in the treatment after impregnation.The Mn removal increased dramatically to 80%. In cook S1169 the EDTAcharge was increased to 1.0% while maintaining the same time andtemperature as in cooks S1166 and S1168; however, as a result of cookS1169 the Mn content was essentially the same as the treatment with 0.5%EDTA, though the Ca removal was markedly greater with the higherconcentration of EDTA. This suggests that under these conditions, forhardwood chips, an increase in EDTA charge does not increase the Mnremoval.

[0052] In cook S1170, the EDTA charge, based upon the results of cookS1169, was reduced to 0.5%, but the chelate treatment temperature wasincreased to 130° C. from 110° C. However, surprisingly, this increasedtemperature resulted in a lower Mn removal compared to the test at thelower temperature and same chelate charge (cook S1168). The reasons forthis are not apparent at this time.

[0053] Finally, a chelate charge of 1.0% and a chelate treatmenttemperature of 130° C. were used in cook S1171. The Mn content of thechips treated in this fashion was the highest in this set of trials, 91%Mn removal. The Ca and Mg removal in the resulting chips was also thehighest in cook S1171.

[0054] The data summarized in Table 9 again confirm that metals can beremoved during pretreatment of the chips with chelating agents.Specifically, the data in Table 9 suggests that the metal removal can bemore effective at higher temperature, for example, 130° C., using anEDTA charge of about 1% on wood, for a treatment time of about 90minutes. Complete cooking trials for hardwood employing chelatetreatment according to the present invention have yet to be completed.TABLE 1 Chip Size and Chip Thickness Distributions Size (mm) % Thickness(mm) % Northern softwood 99-000473 0.0-3.0 0.1 0-2 1.6 3.0-7.0 2.1 2-424.3  7.0-12.7 16.9 4-6 34.7 12.7-25.4 57.0 6-8 22.0 25.4-45.0 21.6 8-10 12.0 greater than 45.0 2.4 greater than 10 5.5 Southern hardwood99-001620 0.0-3.0 0.3 0-2 1.1 3.0-7.0 1.9 2-4 17.3  7.0-12.7 15.6 4-634.5 12.7-25.4 54 6-8 28.3 25.4-45.0 26.6  8-10 12.2 greater than 45.01.7 greater than 10 6.5

[0055] TABLE 2 Summary of treatment conditions and treatment results formetals removal with EDTA for Northern pine. Furnish: Northern softwood99-000473 Cook ID S1159 S1160 S1161 S1162 S1163 Cook Type Special CKSpecial CK Special CK Special CK Special CK EA Charge Impregnation (% onwood NaOH) 9.0 9.0 9.0 9.0 9.0 Treatment (% on wood NaOH) 3.0 3.0 3.03.0 3.0 WL Sulfidity (% AA) 30.0 30.0 30.0 30.0 30.0 EDTA Charge (% onwood EDTA) 0 1.0 1.0 1.0 1.0 Temperature (° C.) Impregnation 95.0 95.095.0 110.0 110.0 Treatment 95.0 95.0 95.0 110.0 110.0 Time (min)Impregnation 30.0 30.0 30.0 30.0 30.0 Treatment 45.0 45.0 90.0 45.0 90.0EA Consumed (% on wood NaOH) Total 7.0 7.8 8.6 9.2 10.1 ImpregnationStage 4.1 4.2 4.5 6.5 6.7 Treatment Stage 2.9 3.6 4.1 2.7 3.4 ResidualEA (g/L NaOH) End of Impregnation 9.9 9.7 9.0 5.1 4.7 End of Treatment10.2 8.6 6.9 5.7 3.9

[0056] TABLE 3 Summary of pulping conditions and pulping results formetals removal with EDTA cooks for Northern pine. Furnish: Northernsoftwood 99-000473 Cook ID S2222 S1182 AL622 AL624 AL625 AE647 Cook TypeCK CK LS LS LS EMCC EA Charge Impregnation (% on wood NaOH) 19.2 19.29.5 9.5 9.5 11.0 Treatment 0.0 0.0 0.0 0.0 3.0 Displacement 1 (g/L NaOH)NA NA 92.5 92.5 92.5 97.8 Displacement 2 (g/L NaOH) NA NA 43.0 43.0 43.029.0 WL Sulfidity (% AA) NA NA 29.2 29.2 29.2 28.2 EDTA Charge (% onwood EDTA) NA 1.0% NA 1.0% 1.0% 1.0% at Imprg. at Imprg. at Treat. atImprg. Temperature (° C.) Impregnation 110.0 110.0 110.0 110.0 95.0110.0 Treatment NA 110.0 NA NA 110.0 NA Cooking 170.0 170.0 158.5 158.5158.5 158.5 EA Consumed (% on wood NaOH) Total 16.1 16.9 17.4 17.9 18.916.4 Impregnation Stage NA NA 7.2 7.6 5.7 8.3 Treatment Stage NA NA NANA 2.4 NA Displacement 1 Stage NA NA 6.5 6.6 7.4 — Co-current Stage NANA 1.2 1.2 1.2 6.0 Displacement 2 Stage NA NA 2.6 2.5 2.2 2.1 ResidualEA (g/L NaOH) End of Impregnation NA NA 6.7 5.3 10.8 8.9 End ofTreatment NA NA NA NA 4.1 NA End of Displacement 1 NA NA 15.6 14.5 17.00.0 End of Co-current NA NA 12.3 11.1 13.6 6.1 End of Cook 8.9 6.7 17.416.8 19.1 18.5 H-factor 1440 1410 1717 1717 1716 1793 Kappa Number 24.924.4 25.5 26.1 22.0 23.6 Viscosity (mPa · s) 32.3 32.1 44.6 51.4 44.554.9 Viscosity/Kappa Number Ratio 1.3 1.3 1.7 2.0 2.0 2.3 Total Yield (%on wood) 45.7 46.7 44.9 45.5 44.8 45.4 Rejects (% on wood) 0.20 0.500.03 0.02 0.01 0.12 Screened Yield (% on wood) 45.5 46.3 44.8 45.5 44.745.3 Knots >2.5 cm (% on wood) 0.10 0.40 0.02 0.01 0.00 0.1

[0057] TABLE 4 Summary of treatment conditions and treatment results forSouthern hardwood. Furnish: Southern hardwood 99-001620 Cook ID S1166S1168 S1169 S1170 S1171 Cook Type Special CK Special CK Special CKSpecial CK Special CK EA Charge Impregnation (% on wood NaOH) 9.0 9.09.0 9.0 9.0 Treatment (% on wood NaOH) 3.0 3.0 3.0 3.0 3.0 WL Sulfidity(% AA) 30.0 30.0 30.0 30.0 30.0 EDTA Charge (% on wood EDTA) 0.0 0.5 1.00.5 1.0 Temperature (° C.) Impregnation 110.0 110.0 110.0 110.0 110.0Treatment 110.0 110.0 110.0 130.0 130.0 Time (min) Impregnation 30.030.0 30.0 30.0 30.0 Treatment 90.0 90.0 90.0 90.0 90.0 EA Consumed (% onwood NaOH) Total 7.4 7.5 7.7 8.4 8.7 Impregnation Stage 7.8 7.7 7.4 7.67.6 Treatment Stage −0.4 −0.2 0.4 0.9 1.0 Residual EA (g/L NaOH) End ofImpregnation 3.5 3.6 4.6 4.1 3.9 End of Treatment 4.7 4.2 3.6 1.6 1.0

[0058] TABLE 5 Metals in chips, brownstock, and black liquor forNorthern softwood. Treatment Treatment Ca Cu Fe Mg Mn Ca Cu Fe Mg MnEDTA Temperature Time Descriptive Repli- (mg/ (mg/ (mg/ (mg/ (mg/ (mg/(mg/ (mg/ (mg/ (mg/ LIMS ID (% on wood) (° C.) (min) ID cate L) L) L) L)L) kg) kg) kg) kg) kg) 99-001208 — — — S1159 1 798.0 0.7 6.9 149.0 105.0CHIPS 2 730.0 0.6 4.3 139.0 89.5 Mean 764.0 0.6 5.6 144.0 97.3 S.D. 48.10.0 1.8 7.1 11.0 99-001206 0.0 95 45 S1159 AI 1 42.9 0.3 2.1 6.0 4.9 239.6 0.3 1.7 4.9 4.7 Mean 41.3 0.3 1.9 5.4 4.8 S.D. 2.3 0.0 0.3 0.7 0.199-001207 S1159 ET 1 41.2 0.3 1.3 4.5 4.9 99-001209 S1159 1 695.0 0.413.4 95.9 57.0 TREATED 2 743.0 0.4 17.5 101.0 66.0 CHIPS Mean 719.0 0.415.4 98.5 61.5 S.D. 33.9 0.0 2.9 3.6 6.4 99-001246 1.0 95 45 S1160 AI 137.6 0.3 1.5 5.2 4.6 99-001246 2 36.2 0.3 1.4 4.1 4.5 Mean 36.9 0.3 1.54.7 4.6 S.D. 1.0 0.0 0.0 0.8 0.1 99-001247 S1160 ET 1 363.9 0.4 2.2 12.912.9 2 363.0 0.3 2.0 12.3 13.0 Mean 363.5 0.4 2.1 12.6 13.0 S.D. 0.6 0.10.1 0.4 0.1 99-001248 S1160 1 316.0 0.3 6.6 92.1 25.6 TREATED 2 362.00.3 7.4 95.1 35.0 CHIPS Mean 339.0 0.3 7.0 93.6 30.3 S.D. 32.5 0.1 0.62.1 6.6 99-001258 1.0 95 90 S1161 AI 1 40.3 0.3 2.2 6.1 5.1 2 38.9 0.32.1 5.2 6.5 3 39.4 0.3 2.3 5.2 5.1 Mean 39.5 0.3 2.2 5.5 5.5 S.D. 0.70.0 0.1 0.5 0.8 99-001259 S1161 ET 1 330.7 0.4 2.9 13.3 15.0 2 336.0 0.32.7 12.8 15.0 Mean 333.4 0.4 2.8 13.1 15.0 S.D. 3.7 0.0 0.2 0.4 0.099-001260 S1161 1 435.0 0.3 6.2 107.0 27.1 TREATED 2 245.0 0.3 8.1 78.519.8 CHIPS Mean 340.0 0.3 7.2 92.8 23.4 S.D. 134.4 0.0 1.4 20.2 5.299-001277 1.0 110 45 S1162 AI 1 46.0 0.3 3.0 7.0 5.3 99-001278 S1162 ET1 300.0 0.4 3.1 16.4 13.5 99-001279 S1162 1 349.0 0.6 47.0 99.9 44.3TREATED 2 281.0 0.5 36.7 77.8 21.0 CHIPS Mean 315.0 0.5 41.9 88.9 32.7S.D. 48.1 0.1 7.3 15.6 16.5 99-001321 1.0 110 90 S1163 AI 1 51.0 0.3 3.47.1 5.7 2 51.0 0.3 3.3 7.1 5.6 Mean 51.0 0.3 3.4 7.1 5.6 S.D. 0.0 0.00.0 0.0 0.1 99-001322 S1163 ET 1 328.0 0.4 4.2 20.5 16.4 99-001324 S11631 163.0 0.4 19.9 71.6 11.3 TREATED 2 144.0 0.4 19.7 66.6 10.0 CHIPS Mean153.5 0.4 19.8 69.1 10.6 S.D. 13.4 0.0 0.1 3.5 0.9 99-002417 1.0 — —S1182 EOC 1 225.0 0.3 47.5 17.6 10.0 99-002421 normal swing CK withS1182 SP 1 183.0 3.1 82.2 130.0 18.7 EDTA at impreg. 2 182.0 3.9 83.8127.0 18.1 Mean 182.5 3.5 83.0 128.5 18.4 S.D. 0.7 0.6 1.1 2.1 0.499-002418 0.0 — — S2222 EOC 1 36.8 0.3 11.4 10.0 8.0 99-002422 normalS2222 SP 1 1184.0 4.7 31.3 140.0 44.9 swing 2 1199.0 10.1 32.4 140.045.7 CK Mean 1191.5 7.4 31.8 140.0 45.3 S.D. 10.6 3.8 0.8 0.0 0.599-002434 0.0 — — AL622 AI 1 55.4 0.4 1.6 6.7 7.4 99-002435 normalLo-Solids ® AL622 EDI 1 55.6 0.3 1.9 6.2 5.4 99-002437 AL622 EOC 1 53.60.4 2.2 8.5 7.9 99-002441 AL622 SP 1 837.0 0.8 13.6 120.0 43.9 2 843.01.4 81.3 127.0 44.1 Mean 840.0 1.1 47.4 123.5 44.0 S.D. 4.2 0.4 47.9 4.90.1 99-002455 1.0 — — AL624 AI 1 298.0 0.3 2.4 18.3 17.5 99-002456normal Lo-Solids ® AL624 EDI 1 199.0 0.3 2.4 13.1 9.5 99-002458 withEDTA at impreg. AL624 EOC 1 150.0 0.3 3.3 11.0 6.9 99-002462 AL624 SP 1287.0 1.2 10.9 90.0 7.5 2 277.0 1.5 11.0 90.0 7.6 Mean 282.0 1.3 11.090.0 7.5 S.D. 7.1 0.2 0.1 0.0 0.0 99-002490 1.0 110 90 AL625 AI 1 44.20.3 1.9 5.3 6.4 99-002491 special Lo-Solids ® with EDTA at AL625 ETI 1287.0 0.6 2.5 16.3 20.2 99-002492 treatment after impregnation AL625 EDI1 192.0 0.3 2.1 12.2 9.3 99-002494 AL625 EOC 1 142.0 0.4 4.2 10.2 6.9 2142.0 0.4 4.4 10.1 7.0 Mean 142.0 0.4 4.3 10.2 7.0 S.D. 79.4 0.1 1.2 3.65.4 99-002498 AL625 SP 1 218.0 0.9 15.4 73.0 5.9 2 220.0 1.9 23.6 72.06.0 Mean 219.0 1.4 19.5 72.5 5.9 S.D. 1.4 0.7 5.8 0.7 0.0 99-003958 1.0— — AE647SP 1 186.2 0.6 12.7 90.3 8.9 EMCC ® with EDTA at 2 183.8 2.813.9 91.5 9.1 impreg. Mean 185.0 1.7 13.3 90.9 9.0 S.D. 1.7 1.6 0.8 0.80.1

[0059] TABLE 6 Metals in chips, brownstock, and black liquor forSouthern hardwood. Treatment Treatment Ca Cu Fe Mg Mn Ca Cu Fe Mg MnEDTA Temperature Time Descriptive Repli- (mg/ (mg/ (mg/ (mg/ (mg/ (mg/(mg/ (mg/ (mg/ (mg/ LIMS ID (% on wood) (° C.) (min) ID cate L) L) L) L)L) kg) kg) kg) kg) kg) 99-001723 — — — S1166 1 1074.0 1.2 9.4 275.3140.7 CHIPS 2 2208.0 1.2 10.2 377.0 88.8 Mean 1641.0 1.2 9.8 326.2 114.8S.D. 801.9 0.0 0.5 71.9 36.7 99-001721 0.0 110 90 S1166 AI 1 26.5 0.41.0 4.0 1.4 99-001722 S1166 ET 1 39.5 0.5 1.1 8.0 2.6 99-001724 S1166 11673.0 0.4 6.6 282.9 49.4 TREATED 2 926.0 0.5 6.4 351.7 106.1 CHIPS Mean1299.5 0.5 6.5 317.3 77.8 S.D. 528.2 0.1 0.2 48.6 40.1 99-001746 0.5 11090 S1168 AI 1 28.9 0.4 1.4 4.4 1.6 2 28.8 0.4 1.4 4.4 1.7 Mean 28.9 0.41.4 4.4 1.6 S.D. 0.1 0.0 0.0 0.0 0.0 99-001747 S1168 ET 1 95.7 0.4 1.88.1 3.0 99-001748 S1168 1 1857.0 0.4 13.2 223.1 21.1 TREATED 2 798.6 0.39.5 228.6 25.7 CHIPS Mean 1327.8 0.3 11.3 225.9 23.4 S.D. 748.4 0.1 2.63.9 3.2 99-001750 1.0 110 90 S1169 AI 1 32.6 0.3 1.4 5.9 1.9 99-001751S1169 ET 1 134.0 0.3 2.0 4.2 5.2 99-001752 S1169 1 390.2 0.4 6.4 242.725.6 TREATED 2 279.6 0.3 6.3 185.1 20.7 CHIPS Mean 334.9 0.3 6.4 213.923.2 S.D. 78.2 0.1 0.1 40.7 3.5 99-001774 0.5 130 90 S1170 AI 1 31.4 0.41.3 5.5 1.9 99-001775 S1170 ET 1 100.8 0.3 2.1 11.6 4.0 99-001776 S11701 1196.0 1.0 11.3 245.0 29.7 TREATED 2 952.5 1.0 11.7 296.8 34.4 CHIPSMean 1074.3 1.0 11.5 270.9 32.0 S.D. 172.2 0.0 0.3 36.6 3.3 99-0017921.0 130 90 S1171 AI 1 27.4 0.3 1.1 4.2 1.5 99-001793 S1171 ET 1 125.00.3 2.5 17.7 5.3 99-001794 S1171 1 320.4 0.5 12.9 178.7 13.4 TREATED 2201.6 0.4 9.6 139.7 6.4 CHIPS Mean 261.0 0.5 11.3 159.2 9.9 S.D. 84.00.1 2.3 27.6 5.0

[0060] TABLE 7 Metal Removal During Kraft Cooking Northern Canadian SWDEffect of EDTA Addition to Post-impregnation Treatment Stage on MetalRemoval. Sample Orig. Chips S1159 S1160 S1161 S1162 S1163 EA Charge (%NaOH) Impregnation 9.0 9.0 9.0 9.0 9.0 Treatment 3.0 3.0 3.0 3.0 3.0 WLSulfidity (% AA) 30 30 30 30 30 EDTA Charge (%) 0 1.0 1.0 1.0 1.0Temperature (° C.) Impregnation 95 95 95 110 110 Treatment 95 95 95 110110 Time (min) Impregnation 30 30 30 30 30 Treatment 45 45 90 45 90Metal Content (mg/kg) Ca 764 719 339 340 315 154 Cu 0.6 0.4 0.3 0.3 0.50.4 Fe 6 15 7 7 42 20 Mg 144 98 94 93 89 69 Mn 97 61 30 23 33 11 % MnRemoval 37% 69% 76% 66% 89%

[0061] TABLE 8 Metal Removal During Kraft Cooking Northern Canadian SWDEffect of EDTA Addition to Impregnation and Post-impegnation TreatmentStages. Cook ID S2222 S1182 AL622 AL624 AE647 AL625 Cook Type CK Ref. CKLS Ref. LS EMCC LS EDTA Charge (%) NA 1.0% NA 1.0% 1.0% 1.0% at Imprg.at Imprg. at Imprg. at Treat. Temperature (° C.) Impregnation 110 110110 110 110 95 Treatment NA 110.0 NA NA NA 110.0 Cooking 170.0 170.0158.5 158.5 2.3 158.5 Kappa Number 24.9 24.4 25.5 26.1 23.6 22.0Viscosity (mPa · s) 32.3 32.1 44.6 51.4 54.9 68.5 Viscosity/Kappa Ratio1.30 1.32 1.75 1.97 2.33 3.11 Total Yield (% on wood) 45.7 46.7 44.945.5 45.4 44.8 Rejects (% on wood) 0.20 0.50 0.03 0.02 0.12 0.01 MetalContent (mg/kg) Ca 1192 183 840 282 185 219 Cu 7.4 3.5 1.1 1.3 1.7 1.4Fe 32 83 47 11 13 19 Mg 140 129 124 90 91 73 Mn 45 18 44 7.5 9.0 5.9 %Mn Removal 53% 81% 55% 92% 91% 94%

[0062] TABLE 9 Metal Removal During Kraft Cooking Southern USA HWDTable. Summary of treatment conditions and treatment results forSouthern hardwood. Sample ID S1166 S1168 S1169 S1170 S1171 Org. ChipsImpregnation Time, min. 30 30 30 30 30 Impregnation Temperature, ° C.110 110 110 110 110 EDTA Treatment Time, min. 90 90 90 90 90 EDTATreatment Temperature, ° C. 110 110 110 130 130 EDTA Charge, % on wood0.0 0.5 1.0 0.5 1.0 Metal Content, mg/kg Ca 1300 1328 335 1074 261 1641Cu 0.5 0.3 0.3 1.0 0.5 1.2 Fe 6.5 11.3 6.4 11.5 11.3 9.8 Mg 317 226 214271 159 326 Mn 77.8 23.4 23.2 32.0 9.9 115 % Mn Removal 32% 80% 80% 72%91%

[0063]FIG. 3 illustrates a modified version of the FIG. 2 embodiment inwhich the vessel 25 is modified. In FIG. 3, the optional cookingcirculation 36 has been deleted. A novelty of the FIG. 3 embodiment isthe dual center-pipe discharge associated with the upper cookingcirculation. In the upper circulation of FIG. 2, when the chelantcontaining liquor is introduced via conduit 31, some of the chelantpasses counter-currently upward and complexes metal-containing compoundsas desired, but some of the chelant passes downward with the downwardflowing slurry, and may be removed via extraction screen 35. It ispreferred that the chelant pass upward and treat the slurry before beingremoved via screen 27. In the embodiment shown in FIG. 3, therecirculation conduit 31 is split into at least two liquor introductionconduits 131 and 132. Chelating agent is introduced to conduit 132 viaconduit 129. Optionally, at least some chelating agent, may also beintroduced to conduit 131 via conduit 130, though typically, little orno chelant is introduced via conduit 130. Conduit 131, containing littleor no chelating agent, discharges as is conventional in the vicinity ofscreen 133. Conduit 132, containing at least some chelating agentdischarges at a point above the discharge of conduit 131. As a result,the chelating agent introduced via the upper outlet of conduit 132passes upward with the counter-current flow of liquid, treats thedown-flowing slurry, and is removed via screen 27 and conduit 28. Theliquid in conduit 131, again, containing little or no chelant, isintroduced in the vicinity of screen 133 and preferably passes downwardinto the cooking process that takes place below screen 133. According tothis embodiment, little or no chelant introduced via conduit 129 is“lost” to the pulping process, and as much chelant as possible ispresent in the counter-current flow between screens 133 and 27 tosequester metal-containing compounds.

[0064] Thus the FIG. 3 embodiment teaches introducing two differentliquor flows into the zone 30, an upper flow which contains chelant andprimarily or substantially exclusively moves upwardly in vessel 25 withupflowing liquid to be removed through screen 27, while the liquorintroduced at 131 primarily or substantially exclusively movesdownwardly with slurry into lower zones of vessel 25.

[0065] Although the present invention is described in relationship tothe kraft process, it is to be understood that it is also applicable toall conventional chemical pulping processes. These include the alkalineprocesses, such as the kraft process (also known as the sulfateprocess), soda process, or soda-AQ process. It is also applicable toacidic processes, such as the sulfite process and its derivatives. Thepresent invention is also applicable to processes employing yield orstrength-enhancing additives, such as AQ, polysulfide, and theirequivalents and derivatives, and surfactants and penetrants. The presentinvention is also applicable to batch pulping processes as well as thecontinuous processes described in the drawings.

[0066] In all of the descriptions above, all narrower ranges within eachbroad range are also specifically disclosed herein. For example aconsistency of 8-20% means 8-12%, 9-16%, 10-18%, and all other narrowerranges within the broad range.

[0067] It will thus be seen that according to the present invention byeffecting an extraction of metals at an early stage in the digestionprocess, preferably right after impregnation, without any chelatingagent treatment, and subsequently performing the chelating agenttreatment, the amount of metals removed from the wood chip slurry duringchemical pulping can be maximized for a given amount of chelant. Whilethe invention has been herein shown and described in what is presentlyconceived to be the most practical and preferred embodiment thereof itwill be apparent to those of ordinary skill in the art that manymodifications may be made thereof within the scope of the invention,which scope is to be accorded the broadest interpretation of theappended claims so as to encompass all equivalent methods and products.

What is claimed is:
 1. A method of producing chemical pulp fromcomminuted cellulosic fibrous material containing metal-containingcompounds, comprising: (a) pressurizing and slurrying the comminutedcellulosic fibrous material containing metal containing compounds, andheating the slurry; (b) digesting the material in the slurry, includingin different stages, impregnating the material with cooking liquor andcooking the material to produce chemical pulp; (c) at an early stageduring digestion, extracting liquid containing some of the metalcontaining compounds therein, and removing the metal containingcompounds from the slurry; (d) after (c), adding a chelating agent tothe slurry to combine with released metal containing compounds in theslurry to produce metal complexes; (e) cooking the material; and (f)removing at least some of the metal complexes formed in (d).
 2. A methodas recited in claim 1 further comprising, after (e), bleaching thechemical pulp with at least one bleaching chemical adversely affected byat least some of the metal-containing compounds removed in (c) and (f).3. A method as recited in claim 2 wherein (c) is practiced to remove atleast about 30% of the Mn in the material from the material.
 4. A methodas recited in claim 3 wherein (c) and (f) are practiced to remove atleast about 90% of the Mn originally in the material from the material.5. A method as recited in claim 1 wherein (c) and (f) are practiced toremove at least about 93% of the Mn originally in the material from thematerial.
 6. A method as recited in claim 1 wherein (c) is practicedafter impregnation, and just before cooking, at a temperature of betweenabout 105-145° C.
 7. A method as recited in claim 6 wherein (d) ispracticed for a time period of between about 30-120 minutes, and with acharge of chelating agent of between about 0.05-10 kg/dry ton ofmaterial.
 8. A method as recited in claim 7 wherein (d) is practiced ata pH of 9 or more.
 9. A method as recited in claim 1 wherein (d) ispracticed after impregnation, and just before cooking, at a temperatureof about 110° C. and for a time period of about 90 minutes, and with acharge of chelant of between about 1-2 kg/dry ton of material.
 10. Amethod as recited in claim 1 wherein the chelating agent is selectedfrom the group consisting essentially of EDTA, DTPA, derivatives andequivalents to EDTA and DTPA, oxalic acid, tartaric acid, and furoicacid.
 11. A method as recited in claim 1 wherein (d) is practiced with acharge of chelating agent of between about 0.05-10 kg/dry ton ofmaterial.
 12. A method as recited in claim 11, wherein (a) through (f)are practiced substantially continuously; and wherein (c) and (f) arepracticed to remove at least about 90% of the Mn originally in thematerial, from the material.
 13. A method as recited in claim 1 wherein(d) is practiced to add chelating agent to the slurry prior to theslurry achieving a temperature of about 120° C. or more.
 14. A method ofproducing chemical pulp from comminuted cellulosic fibrous materialcontaining metal-containing compounds comprising: (a) pressurizing andslurrying the comminuted cellulosic fibrous material containingmetal-containing compounds, and heating the slurry; (b) impregnating thematerial with cooking liquor at a temperature of about 90° C. or more;(c) after (b), extracting liquid containing some of the metal-containingcompounds therein, and removing the metal containing compounds from theslurry; (d) after (c) adding a chelating agent to the slurry to combinewith released metal-containing compounds in the slurry to produce metalcomplexes; (e) cooking the material; and (f) removing at least some ofthe metal complexes formed in (d) and (e).
 15. A method as recited inclaim 14 further comprising, after (e), bleaching the chemical pulp withat least one bleaching chemical adversely affected by at least some ofthe metal ions removed in (c) and (f).
 16. A method as recited in claim14 wherein (c) and (f) are practiced to remove at least about 93% of theMn originally in the material from the material.
 17. A method as recitedin claim 14 wherein (b) is practiced at a temperature of about 95° C. ormore, and wherein (d) is practiced at a temperature of between about105-145° C.
 18. A method as recited in claim 17 wherein (d) is practicedfor a time period of between about 30-120 minutes, and with a charge ofchelating agent of between about 0.05-10 kg/dry ton of material.
 19. Amethod as recited in claim 15 wherein (b) is practiced at a temperatureof about 95° C. or more, and wherein (d) is practiced at a temperatureof between about 105-145° .
 20. A method as recited in claim 19 wherein(d) is practiced for a time period of between about 30-120 minutes, andwith a charge of chelating agent of between about 0.05-10 kg/dry ton ofmaterial.
 21. A method as recited in claim 14 wherein (b) is practicedutilizing alkaline cooking liquor, and wherein (d) is practiced byalkaline cooking.
 22. A method as recited in claim 14 wherein (d) ispracticed to add chelating agent to the slurry prior to the slurryachieving a temperature of about 120° C. or more.
 23. A method asrecited in claim 14 wherein (a)-(f) are practiced in an upright vesselhaving an upper screen and a black liquor extraction screen; and wherein(d) is practiced by adding chelant in a first conduit at a firstdistance below the upper screen, and above the extraction screen, sothat the chelant at least primarily flows with liquid up through theslurry to the upper screen, and wherein (e) is practiced in part byintroducing cooking liquor in a second conduit at a second distancebelow the upper screen, greater than the first distance, so that thecooking liquor flows with the slurry downwardly toward the extractionscreen.
 24. A chemical pulp made by practicing the method of claim 21.